rclone reads every RCLONE_<FLAG> variable as that flag. The step that
installs it set RCLONE_VERSION=v1.75.2 for the download, so its own
"rclone version" ran as "--version=v1.75.2" and failed:
Invalid value when setting --version from environment variable
RCLONE_VERSION="v1.75.2": invalid argument for "--version" flag
Both publish jobs stopped there, before uploading, in every run since the
pin was added. Name the two variables PINNED_RCLONE and
PINNED_RCLONE_SHA256, and have the nightly test refuse any RCLONE_ variable
in a workflow other than the remote's RCLONE_CONFIG_ settings.